Optical head

Abstract

In the optical head, laser beams are converged by the objective lens and projected on the light-receiving surface of the optical disk. The diverged laser beam is reflected by the light-receiving surface, converged by the objective lens into parallel laser beams, and directed to the light shielding plate. Only the laser beam passing through the region spaced from the optical axis is picked up by the light shielding plate, and part of this laser beam is converged by a convergent lens and projected on photo insensitive and first and second photo sensitive regions of the photo detecting unit. The regions of the photo detecting unit are deviated by Δ from a back focal point of the convergent lens, and the optical axis of the convergent lens passes through one of the first and second photo sensitive regions. The photo insensitive region has a width d which is not more than twice the larger of absolute values |h3(Δ, δc)| and |h3(Δ, -δc)| of variables given as follows: h3(Δ, δc)={∓Δ/m+2mδc}(yO/F) h3(Δ, δc)={∓Δ/m-2mδc}(yO/F) where ±δc is a maximum defocusing allowance with respect to the convergent point of the objective lens, m is a magnification of the beam spot formed on the regions of the photodetector, yO is a radius of an exit pupil of the objective lens, and F is a distance between a principal point of the objective lens and the convergent point of the objective lens.

What is claimed is: 
     
       1. An optical head for focusing light beam on a light-reflecting surface comprising: means for generating a light beam;   means for transferring the light beam;   objective lens means for converting the transferred light beam into a convergent light beam with a beam waist, projecting the light beam on the light-reflecting surface and converging the light beam reflected from the light-reflecting surface, said objective lens means having a convergent point and an inherent maximum defocusing allowance ±δc with reference to the convergent point;   means for deflecting the light beam from the objective lens means, depending on the distance between the objective lens means and the light-reflecting surface; and   a photodetector having first and second photo sensitive regions and a photo insensitive region defined therebetween, said photodetector being disposed to cause a part of a beam spot to be formed on the photo insensitive region by the light beam projected on the photodetector when the objective lens is focused and a part of a beam spot to be formed on one of the photo sensitive regions by the light beam projected on the photodetector when the objective lens is defocused on the photodetector over the maximum defocusing allowance ±δc.   
     
     
       2. An optical head according to claim 1, wherein the maximum of defocusing allowance δc is given as follows:   δc=±λ/2[NA].sup.2     where NA is the numerical aperture of the objective lens means and λ is the wavelength of the light beam.   
     
     
       3. An optical head according to claim 1, wherein the beam deflecting means includes a light-shielding plate which transmits only part of the light beam passing through the region apart from the optical axis.